This invention relates to a package made from a thermoplastic resin and having a cavity for holding semiconductor elements such as diodes and transistors and integrated circuits of these semiconductors (hereinafter simply referred to as "semiconductors"), and more particularly to a package structure suitable for erasable programmable read only memory (EPROM).
Materials typifying packages for EPROM are ceramics. In the case when CERDIP-type ceramic packages are used for packaging EPROMS, since a low-melting-point glass is used for adhering an ultraviolet-light transmissible lid, it is necessary to heat a package as a whole at a temperature higher than the melting point of glass (400.degree. to 500.degree. C.) at the time of sealing. When semiconductors to be included in the package have good thermal resistance, there is no problem. When the semiconductors haul poor thermal resistance, the sealing may be possible at such a high temperature however, there arise many problems including a lowered reliability, a large number of steps required for the construction, and a rise in the total cost.
The present inventors have studied the application of packages made from a thermoplastic resin and having a cavity therein for holding semiconductors proposed by the present inventors (Japanese Patent Examined Publication No. 185/83) to EPROM. That is, an EPROM was prepared by forming a window space in a flat molding 1 made from a thermoplastic resin (hereinafter referred to as "flat molding") and having a cavity and a level difference around the window space as shown in FIGS. 2A and 2B, fixing an ultraviolet-light transmissible lid 2 with an adhesive 3 to give a cap shell, interposing a memory element 7 connected to lead frame terminals 5 between a flat molding 4 and the cap shell 1 as shown in FIG. 3, and combining the cap shell and the flat molding 4 with heating. But in such a structure, the adhesive 3 was undesirably peeled off at the interfaces of the cap shell 1 and the adhesive 3 and of the lid 2 and the adhesive 3 due to shear stress caused by difference in thermal expansion coefficients of these materials, i.e., the cap shell 1, the lid 2 and the adhesive 3. There resulted many failures during a gross leak (G/L) test for evaluating sealing properties.
The thickness of adhesive 3 bearable against the shear stress generated at individual interfaces of the cap shell 1 and the adhesive 3 and the lid 2 and the adhesive 3 is calculated as 10 to 370 .mu.m provided that the adhesive has a doubly laminated structure sandwiched by the cap shell and the lid and the elastic modulus of the adhesive is 5 to 24 kgf/cm.sup.2.
In fact, in a process wherein the cap shell and the lid is bonded via the adhesive, when the elastic modulus of the adhesive is large, the adhesive is coated on the cap shell first, and the ultraviolet-light transmissible lid is pressed on the adhesive. In such a case, when the elastic modulus is relatively large, e.g. 10 to 24 kgf/cm.sup.2 and the adhesive thickness is 100 .mu.m or more, which thickness can bear the shear stress, there takes place a problem in that the adhesive flows to the ultraviolet-light transmitting portion due to flow properties of the adhesive depending on the viscosity of the adhesive.
In contrast, when the elastic modulus of the adhesive is relatively small such as 5 to 10 kgf/cm.sup.2, the adhesive layer is destroyed due to the insufficient strength of the adhesive itself under very severe conditions of 2 kgf/cm.sup.2 at 121.degree. C. in an environmental test for evaluating the reliability of a manufactured EPROM.